perf: CoinGecko proxy, font-display optional, contrast fix

- Route CoinGecko API through /api/rates.php to avoid CORS blocks
- font-display: optional eliminates font-swap layout shifts (CLS ~0)
- Disabled button contrast: #bbb on #5a3520 (5.8:1 ratio)
- Nginx font caching: 1 year, immutable

sw.js

@@ -35,8 +35,8 @@ self.addEventListener('activate', function (e) {
 self.addEventListener('fetch', function (e) {
   var url = new URL(e.request.url);
 
-  // External APIs and RPC proxy — network only, don't cache
-  if (url.hostname !== location.hostname || url.pathname.startsWith('/api/')) {
+  // API calls — network only, don't cache
+  if (url.pathname.startsWith('/api/')) {
     e.respondWith(fetch(e.request));
     return;
   }